The secretion of Gonadotropin-releasing hormone (GnRH, also called LHRH) is critical for regulating the pituitary-gonadal axis and ensuring reproductive competence for virtually all vertebrates. GnRH neurons migrate from the olfactory placode across the cribriform plate and olfactory bulb, into the forebrain and hypothalamus during embryonic and early postnatal development. The migratory pathway for these neurons consists of axons that originate in the vomeronasal organ (VNO), and project beyond the olfactory bulb directly into the rostral forebrain. Preliminary studies show that SDF-1/CXCR4 signaling plays a critical role in GnRH neuron migration. Virtually no GnRH neurons reach the ventral forbrain in CXCR4-/- mice. These mice may represent an ideal model for Kallmann Syndrome (KS) or Idiopathic Hypogonadotropic Hypogonadism (IHH). We will analyze the role of SDF-1/CXCR4 in vivo and in vitro studies in mouse and will seek to determine whether individuals with KS or IHH may have SDF-1 or CXCR4 gene mutations. We will also analyze the role of kisspeptin/GPR54 signaling for their ability to influence the mobility and direction of movement of subsets of GnRH neurons. These experiments are proposed to balance studies in vivo using normal and mutant mice with studies in vitro that test particular pathways or mechanisms. The complementary approach is important for ensuring that our results will be relevant to the basic biology of the GnRH neuronal system. In vivo, we will use several lines of mutant mice to analyze their roles in migration of GnRH neurons. In vitro, we will test whether different cues signal through the same or similar mechanisms to drive GnRH neuron movement/migration. We will test mechanisms in individual GnRH neurons by video microscopy using in vitro slice cultures derived from transgenic animals in which green fluorescent protein (GFP) is selectively expressed in GnRH neurons. These studies will lead to an understanding of the complex factors that guide GnRH neurons to their proper destinations in the hypothalamus to control the hypothalamic-pituitary-gonadal axis.